In districts where no sewage system is complete, miscellaneous drain water discharged from each house and restaurant kitchen etc will be directly released under ground. There are put to practical use a variety of types from such simple construction as one in which a box shaped container is divided into two or three communicating chambers arrayed in series by partition barriers. Miscellaneous drain water is passed step by step from the endmost chamber and sediment articles are caused to sediment. The top clear water is released to rivers etc. At the same time periodically, sludge accumulated at the bottom thereof is removed by a vacuum pump, to a combined purifying tank in which night soil and the miscellaneous drain water from kitchens are firstly introduced together to a sedimentation separator tank consisting of a box shaped container. Left stagnant, solid substances are made to sediment and only the top clear flow liquid is transferred to an adjacent aeration decomposition tank equipped with an aeration device. Air bubbles are made to jet, and aerobic bacteria are proliferated, through which decomposition treatment of organic substances is carried out. Next disinfection treatment is carried out in a disinfection tank, and discharge is then made into rivers etc.